The present invention generally relates to the field of three-dimensional imaging techniques in general and, more particularly, to optical tomography and to Optical Coherence Tomography in specific. Tomography in general is a technique for building up a full three-dimensional image of a non-planar object out of a series of two-dimensional image slices through that object. Perhaps the most popular example of this technique is X-Ray Computed Axial Tomography (CAT) scanning where the object is most easily observed from the “sides”; that is, the object generally has only one long dimension and two relatively short dimensions (like a human body) However, another technique, Optical Coherence Tomography (OCT) has become a versatile and useful tool in fields such as biophotonics where the sample being image is typically best observed from the “top”; that is, the object generally has one short dimension and two longer dimensions (like a tissue sample). OCT is a form of range-finding that makes use of the second-order coherence of a classical optical source to effectively section (or level slice) a partially reflective sample with a resolution governed by the coherence length of the source. Sources of short coherence length (and consequently broad spectrum), such as superluminous LEDs or ultrashort laser pulses, are therefore used in OCT. Operationally, the sample (object) is placed in one arm of an interferometer and illuminated through a beamsplitter with short coherence length light. Light is reflected from all depths within the sample (in proportion to the localized reflectivity) and returned towards the beamsplitter. Simultaneously, a mirror in the second arm of the interferometer is also returning a portion of the original beam to the beamsplitter. Recombined at the beamsplitter, the two beams are directed towards one or more detectors, where they are combined with each other. The combined beams coherently interfere only when the optical path lengths to the sample and to the mirror are equal. Thus, the presence and strength of interfering light in a detector is indicative reflectance of the sample at a depth into the object corresponding to the reference mirror position and at the spatial location corresponding to the location of the detector. If an array of detectors is placed in the sensing plane, an entire level-slice can be recorded simultaneously. The full three-dimensional image is built up by scanning the mirror and recording the thus obtained level slices. Alternatively, a single detector is used and spatial scanning over the object produces the level slices.
OCT is subject to two significant limitations; first, the signal of interest is inherently low contrast since the modulated interference signal rides on a non-modulated self interference background. Secondly, optical dispersion in the object reduces the depth resolution capability of OCT; basically dispersion changes the optical path length within the object as a function of wavelength, which in turn makes the different wavelengths that make up the short coherence length light appear to be coming from different physical depths within the object.
Quantum Interference with entangled photons is a recently developed technology. In Quantum Interference two correlated photons are generated from one source. One photon is typically used as a part of a probe beam while the second photon is part of a reference beam. The “experiences” of the probe photon can then be determined by bringing both photons into a Quantum Interference Device. Measurements are performed by adjusting the conditions for interference and observing the rate of coincident detections of the two photons on individual detectors. The theory of Quantum Interference has been described in the scientific literature and is not reproduced here.
The inventors have advantageously used Quantum Interference (QI) in previous inventions such as an apparatus and method for measuring Polarization Mode Dispersion (PMD), as disclosed in U.S. patent application Ser. No. 10/147,149. They now apply the QI phenomenon to address the limitations of OCT. Unlike classical interference phenomena, QI is insensitive to background radiation. Similarly, the QI signal does not have an intensity dependent background; there is none of the self-interference that sometimes dominates the desired cross-product interference in classical interferometry. Similarly, QI can be configured to be insensitive to dispersion effects that can make a classical measurement impossible.
Thus it is desirable to apply QI to Optical Coherence Tomography to perform Quantum Optical Coherence Tomography. One benefit of QOCT is the reduction of the deleterious effect of background light. Another benefit is the ability to tomographically image objects with highly dispersive material. Yet another benefit is an inherent improvement in signal-to-noise ratio that comes from the elimination of the self-interference of the probe and reference beams.